The present invention relates to the machine tool industry and mainly to the manufacture of precision machine tools. More particularly, the present invention relates to an improved spindle unit.
One of the often faced problems in precision machining and in precision measuring processes is high accuracy alignment of a workpiece relative to a spindle""s rotational axis. To achieve on an internal grinding machine, a high level of concentricity between the internal hole and the just ground external surface requires the precision alignment of the workpiece to make the run out of external surface as small as possible. If the workpiece alignment has to be done in two different cross sections, tilting motion must be avoided as well. To obtain the constant diameter of the cutting tool and the desired high level of concentricity between the cutting teeth and the shank of the tool on the tool grinding machine, the blank has to be aligned accurately relative to the rotational axis to avoid both run out and tilting motions. If the required tolerances are within 0.01 mm., the problem can be solved, for example, by usage of a precision collet chuck. The accuracy and repeatability of most precision chucks are within 0.003-0.005 mm. If the required tolerances are of the same range as the chuck""s accuracy or even smaller, the only way is manual adjustment that usually, has to be repeated several times, because manual adjustments take a long time. Obviously, such an approach can not be used for serial production.
Accordingly, it is an object of the present invention to provide a spindle unit that provides a fast and fully automatic way to achieve high precision alignment and to eliminate run out simultaneously in two different cross sections of the workpiece.
The foregoing object is met by the spindle unit of the present invention.
In accordance with the present invention, a spindle unit comprises a body, an external shaft placed within the body and supported for rotation about an axis by one or more bearings and an internal shaft placed into the external shaft and coupled thereto by means of a flexible coupling. The spindle unit further has front and rear journal bearings and a thrust bearing for supporting the internal shaft within the external shaft. Flow control valves with outlets are connected to recesses in communication with the front and rear journal bearings. The flow control valves are used to control the position of the internal shaft relative to the rotation axis by controlling the flow of fluid to the journal recesses. By controlling the position of the internal shaft, the workpiece attached to the internal shaft is automatically self centered.
Other details of the spindle unit of the present invention, as well as other advantages and objects attendant thereto, are set forth in the following detailed description and the accompanying drawings wherein like reference numerals depict like elements.